Issue with asp34-2 ATARI MEGA ST4

[sviru]

I have an issue where 12V is 10.5 with no load and about 6-7V when connected to PCB. Floppy is not working properly. I have replaced all caps and the issue is still there. Any ideas what can be wrong? Thanks for ideas.

psu.jpg

[rubber_jonnie]

I'd suggest possibly Q1 or Q2 is bad if you've recapped and it still has an issue. I'd probably go for Q2 first.

As with any PSU, they can bite you hard, so safety first is always my first piece of advice. Caps, especially the large ones on the input side, can hold a lot of charge and give you a bad or even fatal zap, plus in some cases the heatsinks are live and can do the same.

If you're not sure, just get somebody who is to work on it, and personally I'd just replace with a Meanwell RD50-A like this: Mega ST Meanwell replacement

[DoG]

Are you sure you need 12V for the floppy drive? Only old drives and those with belt needs 12V. Most floppy drives only uses 5V.

[rubber_jonnie]

Are you sure you need 12V for the floppy drive? Only old drives and those with belt needs 12V. Most floppy drives only uses 5V.

Yep, agree with this 100%.

[exxos]

They are only regulated on the 5v. It would be unusual that only 12v would be low under load.

[sviru]

Are you sure you need 12V for the floppy drive? Only old drives and those with belt needs 12V. Most floppy drives only uses 5V.

Unfortunately yes. It is an old one of the first MEGA ST with the "skew front drive facia".

[sviru]

They are only regulated on the 5v. It would be unusual that only 12v would be low under load.

Well this is the case here. Have no idea what could be the possible cause of this. I was wondering if the original fan could be the issue but not very likely.

[rubber_jonnie]

They are only regulated on the 5v. It would be unusual that only 12v would be low under load.

Well this is the case here. Have no idea what could be the possible cause of this. I was wondering if the original fan could be the issue but not very likely.

Have you tried disconnecting the fan or FDD? The fan isn't needed for short periods, and I even have some Megas where there is no fan at all.

[Dead_Cat]

Yes, I had the same issue, my mega fan was basically a dead short, replaced it with a noktua 40mm fan (it fit perfectly) system up and running
.

I got a meanwell 65 as it also has -12 for the STGA.

Oh I have 2 et4000 1 meg VGA isa cards and just got a mach 32 so if anyone needs one just ask.

[sviru]

Yes, I had the same issue, my mega fan was basically a dead short, replaced it with a noktua 40mm fan (it fit perfectly) system up and running
.

I got a meanwell 65 as it also has -12 for the STGA.

Oh I have 2 et4000 1 meg VGA isa cards and just got a mach 32 so if anyone needs one just ask.

I will try this! The fan works fine but indeed maybe this is the issue here since 5v is rock stable.